Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has had a significant impact in the local area networks (i.e., for fiber-to-home uses), which has seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver ever greater volumes of information in the form of data, audio, and video signals to residential and commercial users. In addition, use of optical fibers in home and commercial business for internal data, voice, and video communications has begun and is expected to increase.
The fibers used in local networks are directly exposed to harsh conditions, including severe temperature and humidity extremes. Since prior coatings did not perform well under such adverse conditions, the need existed for the development of higher performance coatings to address the wide and varied temperature and humidity conditions in which fibers are employed. Specifically, these coatings possessed thennal, oxidative, and hydrolytic stability which is sufficient to protect the encapsulated fiber over a long life-span (i.e., about twenty-five or more years).
Optical fibers typically contain a glass core and at least two coatings, i.e., a primary (or inner) coating and a secondary (or outer) coating. The primary coating is applied directly to the glass fiber and, when cured, forms a soft, elastic, and compliant material which encapsulates the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber core when the fiber is bent, cabled, or spooled. Stresses placed upon the optical fiber during handling may induce microbending of the fibers and cause attenuation of the light, which is intended to pass through them, resulting in inefficient signal transmission. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.
Certain characteristics are desirable for the primary coating, and others for the secondary coating. The modulus of the primary coating must be sufficiently low to cushion and protect the fiber by readily relieving stresses on the fiber, which can induce microbending and consequent inefficient signal transmission. This cushioning effect must be maintained throughout the fiber's lifetime.
Because of differential thermal expansion properties between the primary and secondary coatings, the primary coating must also have a glass transition temperature (T.sub.g) which is lower than the foreseeable lowest use temperature. This enables the primary coating to remain elastic throughout the temperature range of use, facilitating differences in the coefficient of thermal expansion between the glass fiber and the secondary coating.
It is important for the primary coating to have a refractive index which is different (i.e., higher) than the refractive index of the cladding. This permits a refractive index differential between the cladding and the primary coating that allows errant light signals to be refracted away from the glass core.
Finally, the primary coating must maintain adequate adhesion to the glass fiber during thermal and hydrolytic aging, yet be strippable therefrom for splicing purposes. Moisture resistance is essential, because moisture also affects the adhesion of the primary coating to the glass. Poor adhesion can result in microbending and/or various sized delaminations, which can be significant sources of attenuation in the optical fiber.
To provide adequate adhesion during thermal and hydrolytic aging, particularly in high temperature and high humidity applications, many primary coating compositions include adhesion promoters which facilitate adhesion, particularly in wet environments, of the primary coating to the glass fiber. A number of suitable adhesion promoters have been described in the art, including acid-functional materials and organofunctional silanes. Of these, organofunctional silanes are preferred, because such silanes are less corrosive and coatings incorporating such silanes tend to better maintain their adhesive properties. Suitable organofunctional silanes which have been described in the art include, generally, amino-functional silanes, mercapto-functional silanes, methacrylate-functional silanes, acrylamido-functional silanes, allyl-functional silanes, vinyl-functional silanes, and acrylate-functional silanes. Exemplary organofunctional silanes are disclosed in U.S. Pat. No. 5,146,531 to Shustack.
Despite the use of adhesion promoters in the art, the need remains for improved adhesion promoters and improved coating compositions for glass fibers. The present invention is directed to overcoming this deficiency in the art.